I have a GTX 680 running at ~100% and a GTX 780 that is taking up the slack (varying from about 30% to one peak at 90%. If you are using our PPBM6 MPEG2-DVD with GPU turned on I went from 24 seconds with the GTX 780 only to 16 seconds with both installed. The GTX 780 is in PCIe slot 1. I guess later I will switch them but I would have thought that PCIe slot 1 would have the largest burden. This is on my older hex-core X58 at 4.2 GHz and 24 GB of RAM.

yes thats the one - i got to 38 seconds with cs6, and CC replicates the same time with one card which is good consitency.

Your time is great... 16 seconds!

so i must decide.. is the 460 slowing the 660 down somehow because surely if its 100% on the 660 it will be doing the same performance as before.. unless somehow memory bandwidth or its being throttled?

I have tested, but in all the changing of GPU's it turns out those results (GTX 480, GTX 580, GTX 680 and GTX 780) were all accidently done in a PCIe x8 v2.0 socket therefore they are a few (probably 3-5) seconds high invaladating much work.

Bill's previous dual-GPU results were done with the second GPU installed in the wrong slot. Most X58 motherboards have three or more PCI-e x16 physical slots, of which only two run at full x16 bandwidth. However, the x16 slots are only PCI-e 2.0 compliant. Plus, the LGA 1366 CPUs do not have the PCI-e hub on the CPU (but instead, the PCI-e 2.0 hub is located on the motherboard, on the IOH that's part of the chipset). Remember, the X58 IOH provides up to 36 PCI-e 2.0 lanes.

On the other hand, the LGA 1155 platform (and by extension the LGA 1150 platform) have only 16 available PCI-e lanes - total (not counting any PCI-e lanes provided by the chipset's PCH). Your Asus P8P67 motherboard does not have a x8/x8 switch at all, so the "secondary" PCI-e x16 physical slot is actually run at up to x4 bandwidth off of the P67 PCH instead of the CPU.

And a switch to a dual-GPU card will not solve the problem imposed by the mainstream CPU platform: The two GPUs will still share the same 16 lanes of PCI-e bandwidth even with a single card (this means that the two GPUs on a GTX 690 will each receive only eight PCI-e lanes of bandwidth).

Thanks for that, confirms my suspicions, i guess im limited to one card now unless i upgrade the one card to the highest end i need a higher end platform for more upgradeability and 2X full 16x cards.

even then... im pretty happy with the speed im getting now with new card and raid but the dual gpu render was really a teaser !

be interesting to somehow benchmark the 16lanes shared on my system.. if i installed both my 285s and did some benchmarking at 16x 4x with both or one that might get to the bottom of it to see how much bandwidth they need?

You actually did this earlier with the GTX 660 Ti. It took a minute and a half in the MPEG-2 DVD test in PPBM6 because that GTX 660 Ti is constrained to PCI-e 2.0 x4 bandwidth (compared to the 38 seconds that you got with that same GTX 660 Ti operating in the correct for your CPU PCI-e 2.0 x16 bandwidth). Plus, the fact that your encoding test times got slightly longer with the second card in the PCI-e x4 slot tells you one thing: A system running two discrete GPUs in an imbalanced x16/x4 mode will never deliver as fast of a performance as a single GPU on that same system no matter what.

Nicol - Two titans would be a great setup! just looked at your motherboard spec on the ppbm test - its a monster! 7 PCI express 3.0 x16 slots.

This is extremely interesting for me as my current setup limits me from using dual cards as the pci lanes are limited and without upgrading the motherboard and cpu really i cant use 2 cards. So the best CUDA card singularly would be my maximum speed increase - Titan over 660ti......?.